As madboy33??, says, it could be starting in safe mode, indicting a possible problem, or it could be that your graphics card's drivers are corrupt and not loading properly. Update your card's drivers to see if that cures the problem, Stour23.

If Jim's restore solution doesn't work, checkout the graphics card's drivers as I suggested earlier, also checkout your refresh rate, make sure it is set properly. Sometimes the settings for the monitor get changed by kids playing with the buttons on the front of the monitor, make sure those are set correctly also.